linux-kernel - Re: [RFC v2 PATCH 4/4] mm: pre zero out free pages to speed up page allocation for __GFP

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Date:   Mon, 4 Jan 2021 14:29:40 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Liang Li <liliangleo@...iglobal.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [RFC v2 PATCH 4/4] mm: pre zero out free pages to speed up page
 allocation for __GFP_ZERO

On Mon, Jan 4, 2021 at 12:11 PM David Hildenbrand <david@...hat.com> wrote:
>
>
> > Am 04.01.2021 um 20:52 schrieb Dave Hansen <dave.hansen@...el.com>:
> >
> > On 1/4/21 11:27 AM, Matthew Wilcox wrote:
> >>> On Mon, Jan 04, 2021 at 11:19:13AM -0800, Dave Hansen wrote:
> >>> On 12/21/20 8:30 AM, Liang Li wrote:
> >>>> --- a/include/linux/page-flags.h
> >>>> +++ b/include/linux/page-flags.h
> >>>> @@ -137,6 +137,9 @@ enum pageflags {
> >>>> #endif
> >>>> #ifdef CONFIG_64BIT
> >>>>    PG_arch_2,
> >>>> +#endif
> >>>> +#ifdef CONFIG_PREZERO_PAGE
> >>>> +    PG_zero,
> >>>> #endif
> >>>>    __NR_PAGEFLAGS,
> >>>
> >>> I don't think this is worth a generic page->flags bit.
> >>>
> >>> There's a ton of space in 'struct page' for pages that are in the
> >>> allocator.  Can't we use some of that space?
> >>
> >> I was going to object to that too, but I think the entire approach is
> >> flawed and needs to be thrown out.  It just nukes the caches in extremely
> >> subtle and hard to measure ways, lowering overall system performance.
> >
> > Yeah, it certainly can't be the default, but it *is* useful for thing
> > where we know that there are no cache benefits to zeroing close to where
> > the memory is allocated.
> >
> > The trick is opting into it somehow, either in a process or a VMA.
> >
>
> The patch set is mostly trying to optimize starting a new process. So process/vma doesn‘t really work.
>
> I still wonder if using tmpfs/shmem cannot somehow be used to cover the original use case of starting a new vm fast (or rebooting an existing one involving restarting the process).

If it's rebooting a VM then file-backed should be able to skip the
zeroing because the stale data exposure is only to itself. If the
memory is being repurposed to a new VM then the file needs to be
reallocated / re-zeroed just like the anonymous case.